To assess the safety and tolerability of two dose levels of R-568 administered orally for 15 consecutive days in patients with secondary hyperparathyroidism (HPT) and end-stage renal disease (ESRD) treated with hemodialysis. Secondary hyperparathyroidism (HPT) is an almost invariable consequence of chronic renal insufficiency. These patients develop diffuse hyperplasia of the parathyroid glands resulting in elevated circulating levels of parathyroid hormone.